The present invention relates to pulse transducers for measuring pulse pressure and velocity in blood vessels. In particular, the present invention relates to applanation tonometers for noninvasively detecting and reproducing pulse pressure waveforms in peripheral vessels and to velocity sensors for noninvasively detecting and reproducing pulse velocity waveforms.
Existing devices provide pulse pressure detectors for noninvasively measuring pulse pressure waveforms in peripheral vessels. The ideal device is an applanation tonometer which flattens the vessel wall and, by negating the effect of the wall tension, measures the internal pulse pressure within the vessel. The tonometer provides continuous measurement throughout the heart's pumping cycle. In actual use, a tonometer faces a certain amount of tissue between the vessel and the sensor. While accurate internal pressures are not obtained, an accurate waveform reproduction of internal pressures is obtained. Because it is noninvasive, the tonometer can provide accurate, continuous blood pressure measurement with negligible risk. Further, tonometry in combination with Doppler flow techniques make possible noninvasive determination of arterial impedance. The use of tonometers is discussed in J. S. Eckerle, "Arterial Tonometry," Encyclopedia of Medical Devices and Instrumentation, pages 2770-2776 (New York 1988); and Michael F. O'Rourke, The Arterial Pulse, pages 25-34 (New York 1992).
In general, an arterial system behaves somewhat like a mechanical plumbing system. Pressure pulses travel down tubes, bounce off obstructions and send reverberations and counterpulses back up the tubes. An applanation tonometer detects analogous pressure pulses in blood vessels. By existing computer analysis techniques, the waveforms are sorted out into their original components to obtain resultant waveforms, including all the complex reflections. The resulting waveforms give information concerning the stiffness of the blood vessel measured, the age of the patient, obstructions in the vessel, and, in many cases, the condition of the heart itself. Applanation tonometry, because it is noninvasive, provides many advantages over invasive techniques. Noninvasive techniques are much less costly and avoid problems such as arterial trauma and infection.
Pulse velocity measurements are also important in studying the dynamics of blood vessels. Velocity waveforms, like pressure waveforms, will yield information useful in determining characteristics of vessels. Existing noninvasive devices have used velocity sensors, such as Doppler crystal probes, to insonate a vessel for pulse detection. By measuring the reflected ultrasound waves, pulse velocity in a vessel may be noninvasively measured. These measurements are generally not quantified and are used merely to detect the presence of a pulse. For example, a pulse velocity detector may be used to follow a vessel down an injured leg in order to detect the spot of no flow where possible surgery may be required. The use of ultrasound for noninvasive measurements is discussed in Bok Y. Lee, "Peripheral Vascular Noninvasive Measurements," Encyclopedia of Medical Devices and Instrumentation, pages 2220-2224 (New York 1988).
Pressure waveform measurements and simultaneous velocity waveform measurements are also useful in analyzing the condition of vessels. For example, simultaneously recorded pressure/velocity relationships may also be utilized to express vascular impedance. Impedance gives valuable information on the presence and intensity of wave reflections and permits a pressure wave to be separated into a forward traveling wave and a backward traveling wave. Analysis of pressure waveforms and velocity waveforms have been conducted using invasive techniques.
What is needed is a noninvasive device that will allow the study of hemodynamics of blood vessels. In particular, there is a need for a noninvasive device that will allow simultaneous measurement of pulse velocity and pressure in blood vessels. Noninvasive measurement techniques, to the extent they yield significant information, are preferable over invasive measurements techniques. Although existing catheters provide simultaneous velocity and pressure measurements, such catheters are highly invasive. Existing noninvasive devices do not provide for simultaneous measurement of pressure and velocity of blood flow at substantially the same location in a single vessel. Such simultaneous measurements would provide useful information in analyzing vessels. No noninvasive device has been developed to meet this need.